588-luthra-jacs-1991-1095 - -, i . 1 A, . J Am Ceram Soc...

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-i, . 1 A, . J Am Ceram Soc 74153 1095-1103 (19911 Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride Krishan L. Luthra* Corporate Research and Development, General Electric Company, Schenectady, New York 1 2301 This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bub- bles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O,, CO, and N1. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen andlor carbon monoxide (or nitrogen) diffusion. These considera- tions indicate that the oxidation of silicon carbide and sili- con nitride should be mixed controlled, influenced both by an interface reaction and diffusion. [Key words: oxidation, silicon carbide, silicon nitride, silicon, bubbles.] I. Introduction ILICA is one of the few oxides that have extremely low S oxygen permeability up to very high temperatures. Conse- quently, there is much interest in using Sic and Si3N4, which are thermally stable up to very high temperatures and form a protective silica film on oxidation. Numerous studies have been conducted on oxidation of these materials.'-23 However, there are wide variations in the observed reaction rates and the morphology of the reaction products. There is a general agreement that the scale is amorphous at the beginning of the oxidation and at low temperatures and tends to crystallize at longer times, at higher temperatures, and in the presence of impuritie~.~*~,~.'~,'~ In this paper, we will concentrate on oxidation of high-purity materials at tempera- tures below -1400"C, where crystallization effects are minimal. Figure 1 shows some of the lowest reaction rates. Several ob- servations emerge from a review of the literature and Fig. 1. (1) The observed rate constants for chemical vapor depo- sition (CVD) Si3N4 are much lower than those for silicon, by factors of 10 to 1000 at -1350" to 11Oo"C, respectively. (2) The observed rate constants for single-crystal Sic are lower than those of silicon and depend on its crystallographic orientati~n.~"~ In addition, Harris12 reports a linear reaction rate law for the (0001) Si face of Sic. Even for polycrystalline Sic, reaction rates might be lower than those for silicon, but the evidence for this is not strong. Costello and TresslersS6 observed reaction rates similar to those for Si at temperatures above -1200°C for sintered and hot-pressed Sic, but Fitzer and Ebi"~'~ observed substantially lower rates than those for Si on powdered Sic, particularly at temperatures less than -1200°C. (3) The activation energies for oxidation of Sic and Si3N4 are of the order of 134 to 498'~~ 330 to 4S9s21 kJ/mol, J. L. Smialek-contributing editor Manuscript No. 198125. Received September 8, 1989; approved Janu- ary 29, 1991: Presented at the 91st Annual Meeting of the American Ceramic Society, Indianapolis, IN, Apr. 24, 1989 (Engmeering Ceramics Division, Paper NO. 3-JII-891.
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This note was uploaded on 12/10/2009 for the course IF PFIS1200 taught by Professor Antonio during the Spring '09 term at Universidade Federal do Rio de Janeiro.

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588-luthra-jacs-1991-1095 - -, i . 1 A, . J Am Ceram Soc...

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